1. Field of the Invention
The present invention relates to a norbornene-based polymer, a film (particularly, a retardation film, a viewing angle-enlarging film, various functional films such as an antireflection film for use in plasma displays, a protective film for polarizing plates) using the same, a polarizing plate, and a liquid crystal display device.
2. Description of the Related Art
Hitherto, since a film using a norbornene-based polymer obtainable by vinyl polymerization of a norbornene-based compound has a characteristic that retardation in a thickness-direction (Rth) is high, the film has been applied to a negative C plate (WO2004/049011). Furthermore, by stretching it, the main chain of the norbornene-based polymer is aligned in a stretching direction and thereby retardation (Re) is exhibited, so that the polymer can be applied to a negative biaxial retardation plate. Namely, a film of the conventional norbornene-based polymer is promising as a retardation film having high Re and Rth.
On the other hand, as displaying modes of liquid crystal televisions have been diversified, necessary retardation films have been also diversified. As one of such films, a retardation film having small Rth or negative Rth has been desired. For example, in so-called in-plane switching (IPS) mode wherein a transverse electric field is applied to liquid crystals, as one means for improving a color tone and a viewing angle at display of black color, it has been proposed to align an optically-compensatory material having a birefringent property as an optically-compensatory film between a liquid crystal layer and a polarizing plate, the material being obtained by combining a film having a positive birefringence and an optical axis in the film plane and a film having a positive birefringence and an optical axis in a normal line direction of the film (JP-A-11-133408).
For the above request, there has been proposed a ring-opened (co)polymer wherein birefringence is substantially not generated, which is obtained by ring-opening (co)polymerization of a norbornene-based monomer which imparts “negative birefringence” to the polymer (JP-A-2005-36201). However, since the negative birefringence is exhibited by monoaxial stretching of the film, it is considered that independent control of Re and Rth is difficult, so that it is highly desired to develop a film of a norbornene-based polymer, which allows only Rth to be small or negative without affecting Re.